Some vaccines are grown in HEK 293 cells. HEK 293 is a cell line taken from an aborted fetus. (HEK -> Human Embryonic Kidney). Many people consider vaccines grown in HEK 293 to be immoral. Chicken eggs are a moral alternative medium.

It doesn't necessarily apply in this case because the Pfizer/BioNTech vaccine was only tested on HEK 293, not grown in it. (Moderna's vaccine, conversely, was grown in HEK 293.) Some people still consider the testing to be immoral. Some don't.